Membrane filters are broadly used in the area of submicron filtration. They typically offer very high filtration efficiencies, and at a specified level can become absolute. Additionally, membranes allow for significant fluid flow through their structures, enabling high per unit throughputs. One drawback of membranes when used in a direct flow through application is that they have very limited filtrate holding capacity. To compensate for this deficiency, separate prefilters can be used to extend the usable life of the membrane. These additional prefilters typically are used to separate out items which are at a larger size than the rating of the membrane, allowing the membrane to apply its limited filtrate holding capacity to the tightest size range at which the filtration operation is occurring.
Membrane laminate filter media have gained increasing recognition for their improved filtering performance. No primary cake is needed to reduce emissions to near zero levels, and nearly all the dust that is collected is removed with every shaking cycle, which helps maintain a consistently low pressure drop across the laminate throughout the life of the bag. Two layer laminates of porous expanded polytetrafluoroethylene (ePTFE) membranes and a variety of different backings have been used. A few examples are: in high temperature pulse jet applications fiberglass backings have been used, in low temperature pulse jet applications polyester felt backings have been used. Felted acrylic or felted PTFE backings have also been used in pulse jet applications where hydrolysis could be a potential problem. In low energy cleaning filtration systems (shakers and reverse air) woven polyester fabric has been used as a backing.
In order for these prefilters to approach the same general level of filtration size as the membrane, they must be processed so as to close their inherent pore size (e.g. by calendering in the case of typical nonwoven or meltblown materials). This additional processing step typically results in a reduction of the flow rate capability of the prefilter, frequently reducing it below the flow rate capability of the membrane, resulting in additional prefilters being required in parallel to accommodate the desired flow rate. Reducing the basis weight and or thickness of the prefilter to improve its flow rate results in a reduction of its filtrate holding capacity.
It would be desirable to have a microfiltration prefilter that could be directly combined with a microporous filtration membrane, that would provide a significant filtration level at the membrane target filtration level without significantly reducing the flow capability of the membrane, and significantly improving the membranes use life by removing a large percentage of the targeted filtrate size and larger items and having significant filtrate holding capacity.